1. Field of the Invention
The present invention relates to a position transducer and an exposure apparatus with the same and, more particularly, to a position transducer and an exposure apparatus with the same, a position transducer being suitable for use with an alignment sensor for an exposure apparatus for exposing a pattern on a mask to a photosensitizable substrate in a photolithographic step for manufacturing semiconductor elements, liquid crystal display elements, image pick-up tubes (CCDs etc.), thin layer magnetic heads and so on.
2. Description of the Related Art
In a photolithographic step for manufacturing semiconductor elements and so on, there has hitherto been frequently employed a so-called stepper for exposing and transcribing a pattern of a reticle as a mask on each shot area on a photosensitizable substrate, such as a wafer, a glass plate or the like, by a step-and-repeat exposure system as an exposure apparatus to be employed for exposing and transcribing a pattern of a reticle acting as a mask onto such a photosensitizable substrate with a photoresist layer coated thereon.
Recently, there is being employed an exposure apparatus of a scanning exposure type, for example, of a step-and-scan system comprising exposing a reticle to a wafer while scanning the reticle and the wafer in synchronization with a projection optical system. Such exposure apparatuses require a particularly high level of precision in alignment of the reticle with each shot area on the wafer due to the fact that multiple layers of circuit patterns are superimposed on the wafer in manufacturing semiconductor elements and so on. Therefore, alignment sensors of various types and systems are employed for such exposure apparatuses.
Among conventional alignment sensors, an alignment sensor of a so-called grating alignment method uses laser beams as a source of alignment and a wafer mark in the form of a grating with its bars or dots arranged periodically. This grating alignment method may be classified by the structure of an alignment optical system or a detection system, a number of alignment beams of light and so on. The grating alignment method may further be broken down into the following types.
A first type of the grating alignment method is of the type comprising allowing one laser beam to strike the whole area of a wafer mark on a wafer, causing two rays of diffraction light generated from the wafer mark to form an image on a reference grating, scanning the wafer mark relative to the reference grating, and detecting the position of the wafer mark on the basis of a variation with a quantity of light transmitted through the reference grating or reflected therefrom.
A second type of the grating alignment method is of the type comprising allowing two laser beams to strike the whole area of a wafer mark on a wafer from the particular yet mutually different directions of the diffraction order and detecting the position of the wafer mark on the basis of the phase of interference light generating in the identical direction from the wafer mark. This is called as an LIA (Laser Interferometric Alignment) type.
The LIA type may be classified into two groups, one group being of a homodyne interference type transferring a wafer mark relative to a static interference fringe formed by two laser beams having no frequency difference and the other being of a heterodyne interference type measuring a phase difference between a signal photoelectrically detecting an interference light (beat light) of two diffraction light components generated from the wafer mark by two laser beams having a slight difference of frequencies therebetween and a reference signal having the same frequency as the frequency difference between the two laser beams and detecting the phase difference as an amount of a pitch-directional deviation of the position of the wafer mark of the grating form from the predetermined reference point.
Where the diffraction light generated from the grating-shaped wafer mark is detected as a signal using a source of monochromatic light in the manner as described hereinabove, the shape of the grating wafer mark may become non-symmetric as multiple thin layers are superimposed on a wafer substrate more and more, or no diffraction light to be detected may be generated for laser beams of certain wavelengths striking the wafer mark due to interference of a thin layer of the surface photoresist coating or for other reasons, or errors in detecting the diffraction light may be caused to occur due to a very faint intensity of the diffraction light generated therefrom. In order to solve those problems and to enable a more accurate detection of the position of the wafer mark, there has been developed an alignment sensor of a heterodyne interference type using a source of polychromatic light having multiple wavelengths.
An alignment sensor of a heterodyne interference type using a light flux with multiple wavelengths is constructed so as to allow two laser beams having different wavelengths to strike the wafer mark of a wafer from a direction of a particular order after the two laser beams with different wavelengths are modified to provide a slight difference in frequency therebetween and to photoelectrically detect the interference light consisting of the multiple wavelength components generated therefrom. The diffraction light of each wavelength component is photoelectrically detected in the form in which it is summed up altogether on a light recipient surface of a photoelectrical detection element, as an example, so that the detection of the position of the wafer mark may be less affected by an influence of interference of the thin layer on the photoresist coating or a deviation of the diffraction light due to an influence of non-symmetric shape between the sectional shapes of the wafer mark.
Further, there is another method of detecting diffraction light using an alignment sensor of a system referred to as an LSA (Laser Step Alignment) system, like the grating alignment system, which comprises forming a laser spot on the wafer by converging one laser beam thereonto, scanning the laser spot relative to the wafer with a wafer mark with dots arranged linearly thereon through a wafer stage, and detecting the position of the wafer mark on the basis of the intensity of diffraction light generated upon passage through the wafer mark beneath the laser spot.
For such conventional alignment sensors, a photomultiplier is employed as a photodetector when a sensitivity of light for detection from the wafer mark is required to be enhanced. Such a photomultiplier, however, may become a cause to induce a variation in temperature or a temperature gradient in the atmosphere surrounding it because it generates heat upon operation in progress. On the other hand, hitherto, the alignment sensor has been provided in the vicinity of an exposing main body portion of an exposure apparatus with the object of making the exposure apparatus compact. There is a risk, accordingly, that the exposing main body portion thereof and the wafer as a recipient that is exposed to light undergo thermal expansion or other transformation, causing faults and irregularities in accuracy of alignment and of exposure to light (accuracy of superimposition). In addition, as a photomultiplier is usually large in size, it is difficult in terms of making the exposure apparatus compact in size as a whole to locate such a photomultiplier nearby the exposing main body portion thereof.
Furthermore, even when there are some cases where a photodiode or the like is employed as a photodetector, a preamplifier and other means provided on such a photodetector may also become a source of generating heat. Where the photodetector generates heat, the heat may cause turbulence of the air surrounding it, resulting in disturbance of the light to be employed for the detection of alignment. This of course may adversely affect the accuracy in the detection of the wafer mark on the wafer.
Particularly, when a source of light having multiple wavelengths is employed for alignment, there are cases in some uses, where such light of different wavelengths is required to be received for detection by different photodetectors. In this case, the number of the photodetectors increases so that the amount of heat generated by them becomes larger leading to the larger possibility of inducing faults and irregularities in precision of exposure. In addition, the source of light for alignment may adversely affect precision in exposure as a source of generating heat, like photodetectors.
Therefore, the present invention has a primary object of providing a position transducer that does not substantially exert an influence of heat upon a substrate to be subjected to detection.
The present invention has another object of providing a position transducer constructed so as to separate a heat source from the exposure apparatus in order to fail to cause the heat generated from the heat source to exert an adverse influence upon a substrate to be subjected to detection.
The present invention has a further object of providing a position transducer that does not or little undergo an adverse influence upon such a substrate due to turbulence of air within the exposure apparatus or for other reasons.
The present invention has a still further object of providing a position transducer that can reduce or minimize an adverse influence to be otherwise exerted upon such a substrate by the generation of heat from a photodetector when such a photodetector having a large calorific power is employed.
The present invention has another still further object of providing a position transducer that can reduce an adverse influence upon such a substrate and enables the detection of a position of the substrate with a high degree of precision, even if a light flux of multiple wavelengths is employed as a light flux for position detection.
The present invention has another still further object of providing an exposure apparatus with such a position transducer disposed therewith, particularly to provide an exposure apparatus capable of excluding an influence from the heat generated from the light for alignment even when there is employed a light source for alignment of a large calorific power, like a light source having multiple wavelengths.
In addition, the present invention has still another object of providing an exposure apparatus with such a position transducer, which does not cause any decrease in precision of alignment even if an optical fiber or the like is employed upon alignment using a light flux of multiple wavelengths.
In order to achieve the objects as described hereinabove, the position transducer according to the present invention is constructed from an illumination optical system for illuminating a light flux for position detection onto a mark for position detection formed on a substrate to be detected, and from a photodetector for receiving the light flux returned from the mark for position detection so as to detect the position of the substrate on the basis of a signal converted photoelectrically by the photodetector, wherein an optical guide is provided for leading the light flux returned from the mark for position detection to the photodetector.
With the arrangement of the position transducer according to the present invention, the substrate to be detected and the exposing main body portion of the exposure apparatus can be separated and isolated from the photodetector acting as the heat source by transmitting the light flux returning from the mark for position detection via the optical guide, thereby excluding such actors as exert an adverse influence upon the detection of the position with high precision, such as thermal expansion or the like.
Further, the arrangement of the position transducer according to the present invention can serve to improve the precision of position detection because the light flux is transmitted to the photodetector via the optical guide and there can be employed a light flux that does not undergo any influence by turbulence of the air surrounding the photodetector.
In this case, it can be taken as an example that, preferably, the light flux for position detection comprises a light flux of multiple wavelengths and the light flux to be returned from the mark for position detection to the photodetector is led for each wavelength through a different optical guide to the photodetector. With this arrangement, an influence of the heat generated from the photodetector can be alleviated or decreased as compared with the instance where light fluxes each of multiple wavelengths are received by the respective photodetectors.
Further, it is preferred that there is employed, as the optical guide, an optical fiber being so adapted as to have its propagation efficiency optimized to the wavelength of the light flux as an object of propagation. The use of such an optical fiber enables the transmission of the light flux of multiple wavelengths at their respectively minimized attenuation ratios, thereby providing, for example, a signal for position detection for each wavelength accurately at an equal SN ratio.
It is additionally preferred that, when the mark for position detection comprises a mark of a diffraction grating form with dots, bars etc. arranged each by a predetermined pitch in the direction of measurement, for example, as shown in FIG. 7, the illumination optical system is constructed by an optical system for illuminating mutually coherent multiple light fluxes as the light flux for position detection from different directions onto the mark of the diffraction grating form, and the photodetector comprises a first photodetector and a second photodetector with each optical guide disposed so as to lead first and second diffraction light separately or independently from each other to the first and second photodetectors, respectively, the first photodetector being arranged so as to receive the first diffraction light consisting of multiple rays of diffraction light generating in a direction parallel to a first direction from the mark of the diffraction grating form, and the second photodetector arranged so as to receive the second diffraction light consisting of multiple rays of diffraction light generating in a direction parallel to a second direction yet different from the first direction from the mark of the diffraction grating form. This construction allows two different kinds of interference light generating in the different directions to be detected independently and separately from each other.
It is furthermore preferred that, when the light flux for position detection comprises a light flux of multiple wavelengths, the light flux of the multiple wavelengths is led for each wavelength to the corresponding illumination optical system through each of the plural optical guides. The manner of transmission of the light flux of each wavelength via each of the mutual optical guides can reduce such an influence to be otherwise exerted by the generation of heat from the light flux for position detection.
Further, the exposure apparatus according to the present invention comprises an exposing main body portion for transcribing a mask pattern on a photosensitizable substrate and a position detection system for detecting a mark of position alignment formed on the photosensitizable substrate, which are constructed in such a manner that the mask pattern is aligned with the photosensitizable substrate on the basis of a result of detection by the position detection system. In the arrangement of the exposure apparatus as described hereinabove, the position detection system comprises a laser light source for generating laser light of multiple wavelengths, an illumination optical system for illuminating the laser light from the laser light source onto the mark for position detection on the photosensitizable substrate, and a light recipient optical system for receiving the light returned from the mark for position detection, the laser light source being disposed separated or isolated from the exposing main body portion of the exposure apparatus. The manner of separating or isolating the laser light source from the exposing main body portion thereof may include, for example, accommodating either of them in a discrete chamber or accommodating both of them with a portion thereof air-conditioned forcibly.
By separating or isolating the laser light source generating laser light of multiple wavelengths, which acts as a heat source in the position detection system, from the exposing main body portion, adverse influences upon the exposing main body portion and the photosensitizable substrate, which may be otherwise exerted by the heat generated from the laser light source, can be excluded. Further, by implementing alignment using laser light of multiple wavelengths, alignment of a high degree of precision can be achieved without any great interference among thin layers of a photosensitizable substrate or a shape of the mark for position detection.
In this case, the exposure apparatus is preferably arranged such that laser light of multiple wavelengths from the laser light source is led to the illumination optical system through a different optical guide for each wavelength. Further, at this time, it is possible to use an optical guide, such as an optical fiber, with its transmission efficiency optimized for each of the wavelengths, so that laser light of multiple wavelengths can be utilized effectively. Further, by synthesizing laser light of multiple wavelengths in the vicinity of the exposing main body portion of the exposure apparatus, the position of the mark for position detection can be detected with high precision.
In accordance with the position transducer and the exposure apparatus according to the present invention, the light recipient optical system is so arranged as to receive, for example, diffraction light generated by the laser light from the mark for position detection in the predetermined direction. This means that the present invention is applied to an exposure apparatus utilizing the alignment system of the grating alignment method or of the LSA method.
In this case, further, the illumination optical system may be so arranged as to illuminate the laser light of multiple wavelengths onto a comfrising mark dots arranged linearly for position detection, and the position of the mark linear dot is to be detected on the basis of an amount of diffraction light received by the light recipient optical system. This means that there is employed an alignment system of the LSA method in which the light flux is of multiple wavelengths.
Other objects, features and advantages of the subject invention will become apparent in the course of the following description with reference to the accompanying drawings.